• Rondeau, Corinne
• Duynhoven, John Van
• Salami, S.
• Mariette, François

Abstract

The measurement of both translational and rotational diffusion within the same matrix can, in principle, pave the way to a better understanding of the microscopic structure of a polymer network since it influences various motion and length scales. Generally, rotational diffusion is sensitive to motions that occur at the nanometer length scale and at the picosecond-to-nanosecond time scale, whereas in translational diffusion measurements, motion is measured over the millisecond-to-second time scale and over distances from tens of nanometers up to hundreds of microns. In this context, NMR techniques have the advantage of simultaneously and non-invasively measuring the translational and rotational diffusion of molecules. Sodium caseinate (SC) was chosen as a model protein to illustrate the potentiality of this complementary approach. SC has been the subject of many physico-chemical studies, because of its widespread use as a thickener, stabilizer and emulsifier of water-based industrial and commercial products. In aqueous solutions, casein molecules are present in the form of fragile star-like aggregates of ~20 nm in diameter containing ~ 4 to 6 gwater /gcasein. SC dispersions behave like model polymeric solutions, with a hyperentanglement regime that begins above 8 g/100 g H2O. Slow acidification of a sodium caseinate solution causes the formation of a gel. In this paper, we present T2 relaxation and self-diffusion coefficient of a rigid dendrimer probe and a flexible PEG probe in dispersions and acid gels of a sodium caseinate polymer system. The results show that the translational diffusion was greatly hindered in SC dispersions, and differences in the diffusion behaviour between PEG and dendrimer probes were observed after the overlap of the SC chains. In this regime, PEG diffused faster than dendrimer, which encountered greater resistance due to its fixed shape and lack of flexibility. On the contrary, the rotational mobility was much less hindered in SC dispersions at all casein concentration ranges investigated and depended on the local protein-probe friction. After coagulation, PEG and dendrimer translational diffusion was found to increase due to structural changes in the casein matrix, which resulted in the formation of large voids, whereas rotational diffusion of the probes was slightly retarded.

Topics

• impedance spectroscopy
• dispersion
• polymer
• mobility
• laser emission spectroscopy
• Sodium
• void
• Nuclear Magnetic Resonance spectroscopy
• dendrimer